1. Field of the Invention
The present invention relates to a power saving, scale free-type electrodeionization water producing apparatus used in various industrial facilities and research facilities in the fields of semiconductor manufacturing, pharmaceuticals, atomic power and steam power generations, food industries, and the like, and to a method of producing deionized water using the same.
2. Description of Background Art
As a method of producing deionized water, a method of passing the water to be processed through ion exchange resins has conventionally been known. This method, however, requires regeneration of the ion exchange resins with chemicals when the ion exchange resins have been saturated with impurity ions. To overcome this operational disadvantage, an electrodeionization water producing method that does not require regeneration with chemicals has been established and put into practice.
The electrodeionization water producing apparatus has a depletion chamber partitioned by a cation exchange membrane on one side and an anion exchange membrane on the other side. The depletion chamber is packed with an ion exchange material. Concentrate chambers are provided on both sides of the depletion chamber with the cation exchange membrane and the anion exchange membrane in-between. The depletion chamber and the concentration chambers are disposed between an anode chamber having an anode and a cathode chamber having a cathode. Water to be processed is supplied to the depletion chamber while applying a voltage. Concentrate water is sent to a concentrate chamber to remove impurity ions from the water to be processed, whereby deionized water is produced.
In recent years, instead of a conventional electro deionization water producing apparatus, in which cation exchange membranes and anion exchange membranes, separated from one another, are alternately arranged and an ion exchange material is filled in every other chamber formed by the cation exchange membrane and anion exchange membrane, thereby forming a depletion chamber, an improved electrodeionization water producing apparatus with a radically remodeled depletion chamber has been developed (Japanese Patent Application Laid-open No. 2001-239270). This improved electrodeionization water producing apparatus includes two small depletion chambers filled with an ion exchange material, this two-chamber depletion chamber formed by a space partitioned by a cation exchange membrane on one side and an anion exchange membrane on the other side, and an intermediate ion exchange membrane disposed between the cation exchange membrane and the anion exchange membrane. Concentrate chambers are provided on both sides of the depletion chamber with the cation exchange membrane and the anion exchange membrane disposed inbetween. The two-chamber depletion chamber and the concentration chambers are disposed between an anode chamber having an anode and a cathode chamber having a cathode. Water to be processed is supplied to one of the small depletion chambers while applying a voltage. The water flowing out of the small depletion chamber is sent to the other small depletion chamber. Concentrate water is sent to the concentrate chamber to remove impurity ions from the water to be processed, whereby deionized water is produced. In the electrodeionization water producing apparatus of this structure, at least one small depletion chamber of the two small depletion chambers may be filled with only one ion exchange material, e.g. an anion exchange material or cation exchange material, or may be filled with a mixture of the anion exchange material and cation exchange material. This makes it possible to reduce the electric resistance according to the type of ion exchange material used and to provide an optimum thickness to obtain high removal performance for silica or carbonic acid, which has not been achieved by conventional electrodeionization water producing apparatuses.
However, since the electric resistance of the concentrate chamber is large in the conventional electrodeionization water producing apparatuses, a rated current requires a high voltage to pass through the concentrate chamber, resulting in a large power consumption. Since regeneration using chemicals is unnecessary as mentioned above, the operation cost for the electrodeionization water producing apparatus is determined by the amount of electric power consumption. If a rectification loss incurred during conversion of AC current into DC current is excluded, the electric power consumption is equal to the DC current between the electrodes multiplied by voltage.
The DC current is determined by the amount and type of ions in the water to be processed and quality required for processed water. In an electrodeionization water producing apparatus, ions adsorbed on ion exchange materials in the depletion chamber must be continuously discharged to the concentration water side by electrophoresis. Supply of current sufficient to move ions is essential for an electrodeionization water producing apparatus to properly exhibit its performance. Therefore, a fixed current operation, in which a prescribed level of current value above the minimum current value necessary for the operation is maintained, is usually adopted for an electrodeionization water producing apparatus. Hence it is impossible to save power consumption by reducing this prescribed level of current value.
On the other hand, the voltage is a sum of electric potentials created by electric resistances of the electrode chambers, concentrate chambers, and depletion chambers disposed between both of the electrodes, and the ion exchange membranes separating them. The voltage thus depsides upon the performance of the ion exchange materials and ion exchange membranes forming these depletion chambers, types of counter ions, and type and amount of ions contained in the chamber water, and the like. Among these, the electric resistance of the concentrate chambers is significant as compared with other structural components of the electrodeionization water producing apparatus. Specifically, not only is the electrode chamber usually provided only at each end of the electrodeionization water producing apparatus, but also its internal ion strength is comparatively high. A plural number of ion exchange membranes and depletion chambers are usually disposed between the electrodes. However, since the ion exchange membranes are electro-conductive solids each having ion exchange groups and the depletion chamber also has an electro-conductive solid (ion exchange material) filled therein, these components have comparatively small electric resistance. In contrast, the concentrate chambers are also disposed in plural numbers between the two electrodes and in the conventional electrodeionization water producing apparatuses, the concentrate chambers are not filled with conductive materials. Therefore, the conductivity of the concentrate chambers depsides only on the ions possessed by the chamber water, giving rise to a high electric resistance. This has become a major factor of an increase in the overall electric resistances of the apparatus.
In addition, the conventional electrodeionization water producing apparatuses have a problem of forming scale such as calcium carbonate and magnesium hydroxide in the concentrate chamber when the water to be processed has a high hardness. If scale is formed, the electric resistance increases in the scaled part, resulting in decreased current flow. To cause the current to flow in a quantity equivalent to that flowed when there is no scale, the voltage must be increased, thereby increasing the power consumption. In addition, the current density varies according to the scaled area in the concentrate chamber, which leads to a non-uniform current in the depletion chamber. If the deposits of scale increase further, the pressure difference of flow is elevated, followed by a further increase in the voltage. The current decreases when the voltage exceeds the maximum voltage value for the apparatus. In this instance, the current necessary for the removal of ions cannot flow, giving rise to a deterioration of the quality of processed water. What is worse, grown scale invades the inside of the ion exchange membrane and ultimately breaks the ion exchange membrane.
In an effort to decrease the electric resistance originating from the concentrate chamber and to prevent scale formation, an electrodeionization water producing apparatus having a concentrate chamber packed with ion exchange materials has been proposed. As an ion exchange material for packing the concentrate chamber and the form of filling such an ion exchange material, common bead ion exchange resins of styrene-divinylbenzene copolymer containing sulfonic groups and quaternary ammonium groups introduced therein is known, for example. Japanese Patent Application Laid-open No. 2001-225078 discloses an electrodeionization water producing apparatus having organic porous anion exchange material layers spread in the concentrate chamber. As examples of the organic porous anion exchange material, the patent application discloses an organic fine porous membrane made from polyolefin or fluorine-containing resin with anion exchange groups introduced by radical polymerization or radiation-initiated polymerization, a material derived from said organic fine porous membrane prepared by impregnating it with a water-soluble polymer having anion exchange groups, followed by immobilizing the water-soluble polymer by heat treatment or the reaction of the water-soluble polymer with said organic fine porous membrane, and an organic porous anion exchange material prepared by bonding a bead anion exchange resin with a binder containing low density polyethylene as a main component.
The patent specification claims that scale formation can be prevented in such an electrodeionization water producing apparatus having a concentrate chamber packed with an ion exchange material because the electric resistance is decreased due to conductivity of the ion exchange material and also because local mixing of calcium ion or magnesium ion with carbonate ion or hydroxide ion in excess of the solubility product constant due to uneven distribution of ions in the concentrate chamber can be prevented.
However, a concentrate chamber packed with the above-described conventional ion exchange resins or organic porous anion exchange material layers cannot exhibit a sufficient electric resistance reduction effect due to insufficient conductivity of these ion exchange materials. For this reason, the thickness of the concentrate chamber is limited to a certain level, which precludes satisfactory prevention of local mixing of calcium ion or magnesium ion with carbonate ion or hydroxide ion in excess of the solubility product constant, which causes scaling.
Generally, in an electrodeionization water producing apparatus, ions are discharged from a depletion chamber to the next chamber through an ion exchange membrane by electrophoresis, whereas ions coming in the concentrate chamber are precluded from moving by electrophoretic migration by the ion exchange membrane and are only discharged from the outlet port together with effluent water. When an ion exchange material is packed in such a concentrate chamber, almost all ion exchange groups possessed by the ion exchange material form ion pairs with impurity ions other than hydrogen ion and hydroxide ion during steady operation. Since the conductivity of ion exchange material is greatly affected by the mobility of counter ions and the hydrogen ion and hydroxide ion have a mobility several times greater than other ions, the electric resistance of an ion exchange material in which the counter ions are impurity ions other than the hydrogen ion and hydroxide ion is significantly higher than the electric resistance of other ion exchange materials in which the counter ions are hydrogen ions and hydroxide ions. In contrast, in the depletion chamber most counter ions for ion exchange groups are hydrogen ions and hydroxide ions during normal operation. The electric resistance of the depletion chamber is maintained comparatively low due to the mobility of these ions.
Specifically, in an ion exchange material packed in the concentrate chamber in which almost all counter ions are impurity ions during normal operation, no electric resistance reduction owing to having hydrogen ions and hydroxide ions as counter ions can be expected. Therefore, the ion exchange material itself must have high conductivity. However, the above-mentioned conventional ion exchange resins and organic porous anion exchange materials are not designed to have conductivity themselves. Specifically, spherical ion exchange resins with a diameter of 0.2 to 0.5 mm made from a styrene-divinyl benzene (DVB) copolymer with a sulfonic acid group (R—SO3−H+) introduced as a cation exchange group and a quaternary ammonium group (R—N+R1R2R3) as an anion exchange group have been used heretofore as typical ion exchange resins. In these ion exchange resins, current transmission (or transmission of ions) in ion exchange resin beads is effected at low-resistance via ion exchange groups uniformly and densely dispersed in the polymer gel, whereas in the interface of ion exchange resin beads, the flow of ions is concentrated on the interface due to a long migration distance of the ions in water during movement of ions and also due to a small contact area between the beads because of the spherical form of the resin beads, thereby precluding current transmission and causing an electric resistance to increase. Among the aforementioned organic porous anion exchange materials disclosed in Japanese Patent Application Laid-open No. 2001-225078, the organic fine porous membrane with anion exchange groups introduced by radical polymerization or radiation-initiated polymerization and the material derived from the organic fine porous membrane prepared by impregnating it with a water-soluble polymer having anion exchange groups, followed by immobilization of the water-soluble polymer by heat treatment or the reaction of the water-soluble polymer with the organic fine porous membrane have no ion exchange groups in the organic fine porous membrane functioning as a base, but merely contain anion exchange groups introduced on the surface. Therefore, migration of ions is limited to near the surface of the ion exchange material, providing no sufficient electric resistance reducing effect. In the case of the organic porous anion exchange material prepared by bonding bead anion exchange resin with a binder containing low density polyethylene as a main component, ion exchange groups are not present in the binder portion or, even if present, the structures of the binder polymer matrix and ion exchange groups in the binder portion are different from those in the ion exchange resin portion. In addition, the density of the ion exchange groups in the binder portion is lower than that of the ion exchange resin portion. The material is not a homogeneous ion exchanger as a whole. Therefore, migration of ions still remains inhomogeneous and improvement with regard to electric resistance is not necessarily sufficient.
Since the concentrate chamber formed by packing these conventional ion exchange materials does not exhibit a sufficient electric resistance reducing effect in this manner, the thickness of the concentrate chamber must be minimized, giving rise to a problem of insufficient scale prevention effect The scale prevention mechanism in concentrate chambers packed with ion exchange materials is as follows. Specifically, in the area of the concentrate chamber packed with anion exchange materials, anions having permeated through the anion exchange membrane do not move into concentrate water, but pass through the high conductivity anion exchange material and move to the cation exchange membrane, where anions migrate into the concentrate water for the first time. In the same manner, in the area of the concentrate chamber packed with cation exchange materials, cations having permeated through the cation exchange membrane do not move into the concentrate water, but pass through the high conductivity cation exchange materials and move to the anion exchange membrane, where cations migrate into the concentrate water for the first time. For this reason, the high concentration area for calcium ions and magnesium ions and the high concentration area for carbonate ions and hydroxide ions in the liquid causing scale in the concentrate chamber are respectively in the neighborhood of the anion exchange membrane and cation exchange membrane which are located at both sides of the concentrate chamber, whereby mixing of these ions exceeding solubility product constant can be avoided and formation of scale is prevented. As is clear from this scale prevention mechanism, in order to achieve a sufficient scale prevention effect in the concentrate chamber, the distance of the anion exchange membrane and cation exchange membrane located at both sides of the concentrate chamber, specifically, the thickness of the concentrate chamber, must be sufficiently secured. However, since the conventional ion exchange materials filled in the concentrate chamber do not exhibit a sufficient electric resistance reducing effect as mentioned above, the thickness of the concentrate chamber cannot be made as large as desired, giving rise to a problem of insufficient scale prevention effect.
An object of the present invention is therefore to provide an electrodeionization water-producing apparatus having a scale-free concentrate chamber, in which the problems of electric resistance reduction and scale formation have been solved by remodeling the structure of the concentrate chamber in the electrodeionization water producing apparatus so that the electric resistance can be reduced and formation of scale in the concentrate chamber can be prevented during a continuous operation over a long period of time, and a method of producing deionized water using the same.